Sorry, It can not work at all.
function admin_menu() is a method of class at the plugin, which permission you wish to modify.
You can not just take out method from the class, put it somewhere else and wait that it will work.
array( &$this, "admin_screen" ) is the reference to the plugin PHP object. Your theme functions.php know nothing about it.
Code fragment from my post is useful if you place it to your own plugin or prevent user from use some functionality. In this case it is useless.
Please think, if it could be possible to achieve your goal just placing 3-5 lines of code to the functions.php, why I didn't share this information with you?
Because of I with my knowledge of PHP and WordPress API do not know how to do it without modifying plugin code directly or use other plugin, created especially for that.
Thanks for your interest and using User Role Editor.